300 ESTUARY OF THE RIVER SEVERN. March 24, 1846. SIRJOHN KENNIE, President, in the Chair. NO.736. " Onthe Estuary of theRiver Severn." By William Parkes, Assoc. Inst. C.E.* The author having been engaged in making observations on dif- ferent parts of the Severn, below Gloucester, for Mr. Walker, when projects for the erection of bridges and other works connected with proposed railways were referred to him by the Admiralty, has laid them before the Institution, in pursuance of the advice often given by Mr. Walker,when President, that the younger members of the profession should communicate the result of their observations when engaged upon surveys, even though the information so obtained may not be perfectly novel ; and this, he hopes, will be sufficient apology for offering no more than a mere sketch, which is all that surveys, extending only over a few weeks, and made for particular purposes, can enable him to give. The river Severn takes its rise at the foot of Plynlymmon, in the county of Montgomery. At Welshpool, inShropshire, it becomes navigable;and gradually increasesin dimensions, in its course of 140 miles, between that point and Gloucester. About a mile above Gloucester the river separates into two chan- nels, which meet again about a mile below the city. One channel is nearly direct ; the other, which is smaller, passes by Gloucester, and is about a mile longer. For about 10 miles below Gloucester (as far as Longney) the river isgenerally deep and narrow; in some partsthere is a depth of nearly 20 feet at average low water. In general, the surface of the water is below the level of the adjoining land; but the suddenfloods, to which the Severn is particularly subject, owing to the hilly cha- racter of the country through which it passes, frequently raise the water above the level of the land, which would be inundated, but for the embpnkments on each side; From Longney, the river assumes the character of a tidal estuary, rather than of afresh-water stream. The effect of the land-floods, though still verysensible, is much diminished, and the rise of the tide, which above is of comparatively little importance for the navi- gation, is here the only means by which it is carried on. * The discussion of this paper extended over four evenings, but an abstract of the whole is given consecutively, for the sake of facility of reference. Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. ESTUARY OF THE RIVERSEVERN. 30 1 As far as Longney, the inclination of the surface of the water is slight, and follows the general rule observed in other rivers, of gra- dualdiminution inits declivitytowards the outlet, the freer and more enlarged channel allowing of a less surFace velocity, and, con- sequently, less fall, with a much greater discharge. The fall between Gloucester and thispoint is about lt inch permile. It is pretty regular; for although there are some obstructions of rocks and shoals, they are so low as not to offer any great impediment to the course of the stream, when there is an average quantity of water in the river. Below Longney the river becomes of greater width ; its low-water channel is irregular and shallow, so that in many places there is not more thanone foot in depth,and it frequently shifts among the extensive shoals of mud and sand, of which the bed of the river pre- sents an almost continuous series. Thestream is ratherrapid at all times, but varies considerably, according to the quantity of fresh water coming down. The fall in the surface here increases to a most remarkable extent. For the first 2 miles, from Longney to Framilode, the fall is 9 inches pe.r mile; for the next 8 miles, to Hock Crib, 14 inches per mile; thence 5 miles to Sharpness Point, 24 inches per mile. Upon the supposition, that below Sharpness Point the high- water line is comparatively level, and calculating from the difference of the lift of the tide, the fall in the next 12 miles, to the Old Pas- sage, is about 15 inches per mile, below which point it becomes nearly level;for at Kingroad, 8 miles lower down, the lift is nearly the same as at the Old Passage. This lowerportion of theSevern, from Longney to theOld Passage, is, in fact, a great natural weir, which keeps the water of the upper portion of the river at a higher level than would be other- wise due to its distance from the sea. The greater part of the shipping avoid this natural weir, by taking the Gloucester and Berkeley Ship Canal, which, being 13 feet 6 inches above low water at Gloucester, carries that level down to Sharpness Point, 16 miles distant, or 25 miles in the course of the river, where the low water is 34 feet below it. What has been said will give some idea of the capacity of the Severn, for draining a large portion of the central counties of Eng- landand Wales. The area of countrydrained by this river and its tributaries above the Wye, is 4,400 square miles. That drained by the Thames is 6,000 square miles; but from the Severn passing through a hilly country, a larger proportion of the water is removed by drainage, and asmaller proportion by evaporation,thau in the district of the Thames, theTrent, andother largerivers of this country. Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 302 ESTUARY OF THE RIVER SEVERN. The navigation of the Severn is equally important, and, perhaps, more remarkable, as it depends entirely upon thetide, from Kingroad to near Gloucester. Viewed in reference to its tides, the portion of the river now under notice may be considered part of the Bristol Channel ; for it is from the peculiar form andposition of the Channel,. that it derives its facilities for navigation. The Bristol Channel is a funnel-shaped arm of the sea, directly open from the westward to the Atlantic. The width at the mouth, from St. Gowan's Head to Hartland Point, is 43 miles; and it con- tracts, gradually and pretty regularly, in a distance of 90 miles, to a breadth of 7 miles at Kingroad. The consequence of this form is, that although the tide-waveon the coast of Pembrokeshire is not more than 24 feet high, the water, running up with considerable momentum, is, as it were,choked, and is lifted above its natural sea level. Thus the lift of a high tide at Kingroad is 47 feet ; at the Old Passage it is 45 feet; and it has been generally believed, that at Chepstow, which is upon the Wye, 3 miles above its mouth, the lift of tide was most ex- traordinary, having been stated to be56 feet, and some accounts have raised it as high as even 70 feet; but. the author, by Mr. Walker's direction, measured a tide, and found it to correspond exactlyat high-water level with the same tide at the mouth of the Wye ; the river fall at low water being at the latter spot 2 feet 8 inches. The lift at Chepstow was 37 feet 10 inches, and at Beachley,near the confluence of the Wye with the Severn, it was 40 feet 6 inches; so that atide, corresponding to a45-feet tideat the AustPassage, would rise 42 to43 feet, and an extraordiuary tide 4 to 5 feet higher at each place. At a distance of 2f miles below the confluence of t.he Wye, are the English Stones,rockya shoal of greatextent. The low-water channel throughit, which is called :he Shoots, is narrow and tortuous. Immediately above this is the Dun Sand, which lies in the middle of the river, having a channel on each side of it. .The width from shoreto shore, ishere 2 miles.Above the Wye is the promontory of Beachley, a high rocky headland, jutting into the middle of the river, and contracting its width to little more than a mile. At this point is situated the Old, or Aust Passage, a ferry which is on the main road from Bristol, and the southern and south-western parts of England, to South and North Wales and Liverpool. From the con- traction of the river at this spot, the bed is nearly clear of mud and sand; in fact, with the exceptionof a smallbank called the Dod Sand, the bottomis hard limestone-rock. The main set of the tide Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. ESTUARY OF RIVERTHE SEVERN. 303 is through the channel for the navigation, which is close to the north, or Beachley shore. Thefirst set of flood tideis over towards the southern shore, owing to the influence of the land waterfrom the Wye, and when there is a heavy flood in that river, this direction of the current lasts longer, until, in fact, its power is overcome by the greater volume and force of the tidal water, when the latter takes a more direct course. The rise of a good spring tide at this point is, as has been before stated, about 45 feet; the flood lasts 44 hours, and the ebb 73 hours ; the flood-tide current is very rapid, generally about 64 miles an hour. There is scarcely any slack water either at high, or low water ;but the current turns, and thewater begins to rise, or fall, immediately after it has ceascd ebbing, or flowing.